Coolant Distribution System For A Rack Having A Rear-Door Heat Exchanger

ABSTRACT

One embodiment of the invention provides a rack assembly for cooling a computer system. A rack provides support for one or more columns of heat-generating electronic devices. Device fans move air from an air inlet side of the rack through the devices and through an air outlet side of the rack. A unitary door has a support frame spanning the air outlet side of the rack and hingedly coupling the door to a rear vertical edge of the rack. The unitary door includes an air-to-liquid heat exchanger panel spanning an air outlet passage inside the support frame so that substantially all of the air passing through the air outlet side of the rack must pass through the heat exchanger panel. The air outlet passage has a cross-sectional area that is substantially equal to or greater than the cross-sectional area of the one or more columns of heat-generating devices. A coolant circulation system includes a coolant distribution unit for chilling liquid coolant, a supply hose placing the coolant distribution unit in fluid communication with an inlet manifold of the air-to-liquid heat exchanger, and a return hose placing an outlet manifold of the air-to-liquid heat exchanger in fluid communication with the coolant distribution unit. The supply hose and return hose are routed outside the cross-sectional area of the air outlet passage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to cooling computer systems, and more particularly to the circulation of coolant to a rear door heat exchanger on a rack.

2. Description of the Related Art

Computer systems use electrical energy and produce heat as a byproduct of electrical resistance. Rack-based computer systems include many rack-mounted components in a high-density arrangement, which can produce a considerable amount of heat. Excess heat must be removed from the rack to control internal temperatures and to maintain system reliability, performance, and longevity. In a rack-based computer system, fans move cool air through the rack to remove the excess heat and cool the components. The heated exhaust air must then be transported to a computer-room air conditioner (“CRAC”) that cools the air before returning the cooled air to the computer room.

The racks in a computer room are commonly arranged in an organized hot-aisle/cold-aisle layout to minimize the likelihood of appreciable volumes of heated exhaust air from directly re-entering the racks. A hot-aisle/cold-aisle layout may include alternating hot aisles and cold aisles, with the front of each rack sharing a cold aisle with one adjacent rack and the rear of each rack sharing a hot aisle with another adjacent rack. The CRAC supplies the cooled air to the cold aisles. The air from the cool aisle is drawn into the front of each rack and the heated air is exhausted through the rear of the rack to the hot aisle. The heated exhaust air recirculates through the CRAC to be cooled and returned back to the cold aisles.

SUMMARY OF THE INVENTION

One embodiment provides a rack assembly with a rear-door heat exchanger for cooling a computer system. A rack provides support for one or more columns of heat-generating electronic devices. Device fans move air from an air inlet side of the rack through the devices and through an air outlet side of the rack. A unitary door has a support frame spanning the air outlet side of the rack. A hinge pivotally couples the door to a rear vertical edge of the rack. The unitary door includes an air-to-liquid heat exchanger panel spanning an air outlet passage inside the support frame so that substantially all of the air passing through the air outlet side of the rack must pass through the heat exchanger panel. A coolant circulation system includes a coolant distribution unit for chilling liquid coolant, a supply hose placing the coolant distribution unit in fluid communication with an inlet manifold of the air-to-liquid heat exchanger, and a return hose placing an outlet manifold of the air-to-liquid heat exchanger in fluid communication with the coolant distribution unit. The supply hose and return hose are routed outside the cross-sectional area of the air outlet passage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective front view of a double-wide rack with an integrated rear-door heat exchanger and a coolant circulation system according to one embodiment of the invention.

FIG. 2 is a schematic partial assembly view of the double-wide rack illustrating how various modules may be supported on the double-wide rack.

FIG. 3 is a rear elevation view of the double-wide rack with a partially cut-away view of the rear-door heat exchanger.

FIG. 4 is a perspective view of the rear-door heat exchanger from below.

FIG. 5 is a sectioned view of a portion of the fin tube assembly used in the rear-door heat exchanger.

FIG. 6 is a plan view of the double-wide rack with the rear-door heat exchanger in a closed position.

FIG. 7 is a plan view of the double-wide rack with the rear-door heat exchanger pivoted to an open position.

FIG. 8 is a rear elevation view of the rack illustrating an optional routing of the hoses from the ceiling to the rack.

FIG. 9 is a rear-elevation view of the rack illustrating an alternative routing of the hoses from the ceiling.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of the invention is a double-wide rack with an integrated liquid-coolant rear-door heat exchanger having a coolant circulation system that provides chilled coolant to the rear-door heat exchanger with minimal interference with airflow through the rear-door heat exchanger. The double-wide form factor of the rack gives the accompanying rear-door heat exchanger a larger surface area to achieve a cooling performance on the order of 100% heat removal. A plurality of modules is mounted in a chassis from the front of the double-wide rack. The modules are organized within two sets of high-power zones and adjacent low-power zones. The high-power zones contain primarily processor-intensive modules such as compute modules. Modules in the adjacent low-power zones are low-power devices such as network switches and power distribution units (PDUs) for supporting the modules in the high-power zones. The chassis include relatively large-diameter fans that drive airflow through the double-wide rack from the front to the rear. As a result, the capacity of the chassis fans is sufficient to drive airflow exhausted from the double-wide rack through the rear-door heat exchanger, without the use of external “booster” fans. A coolant supply hose and return hose are routed from a coolant distribution unit to respective inlet and outlet manifolds on the rear-door heat-exchanger in a manner that maximizes airflow through the rear-door heat exchanger and which maximizes the circulation of the coolant. The coolant supply hose and return hose may be routed underneath the base pan of the rack, perpendicular to a hinge axis of the rear-door heat exchanger. The coolant supply hose and return hose may be routed upward to the rack from a raised floor of the data center. Alternatively, the coolant supply hose and return hose may be routed downward to the rack from the ceiling, in which case the rack may be placed on a more economical slab floor. The inlet and outlet manifolds are spaced from the high-power zones, to avoid interfering with airflow through the rear-door heat exchanger from the high-power zones.

FIG. 1 is a perspective front view of a double-wide rack with an integrated rear-door heat exchanger 100 having a coolant circulation system 116 that provides chilled coolant to the rear-door heat exchanger 100 with minimal interference with airflow through the rear-door heat exchanger 100. The double-wide rack 10 supports a plurality of modular electronic components (“modules”) and provides access to the modules primarily from the front 12 of the double-wide rack 10. The various modules are arranged with comparatively high-power modules in a first set of two vertical columns referred to as “high-power zones” 16, 18, alternating with a second set of comparatively low-power modules in adjacent vertical columns referred to as the “low-power zones” 20, 22. The modules in the high-power zones 16, 18 include processor-intensive modules, such as blade servers and other “compute modules” having a motherboard and one or more processors. The high-power modules consume a large amount of electrical power as compared to the low-power modules, and generate a correspondingly large amount of heat that must be removed from the double-wide rack 10 by the rear-door heat exchanger 100. The modules in the low-power zones 20, 22 are typically devices for supporting the high-power modules, such as network switches for providing network connectivity and power distribution units (PDUs) for distributing power to the modules in the high-power zones 16, 18. Modules in the high-power zones 16, 18 may each have a power consumption above a selected power threshold, e.g. 100 watts, while modules in the low-power zones 20, 22 may each have a maximum power consumption below the same or another selected power threshold, such as 60 watts. Thus, the low-power modules typically contribute a relatively small amount to the overall heat production of the double-wide rack 10. The term “double-wide” is applied to the rack 10 because the modules are arranged in two vertical groupings 15, 17 of complimentary high-power and complimentary low-power zones. This double-wide rack configuration gives the rack 10 a unique form factor having approximately twice the width of a conventional rack having a single column of high-power modules.

A plurality of fans inside the rack 10 generate airflow through the rack 10 to cool the various modules mounted in the rack 10. The fans drive the airflow through the rack 10 from the front of the rack 12 to the rear 14 of the rack 10 and through the rear-door heat exchanger 100. The double-wide rack configuration increases the area of the airflow through the matching rear-door heat exchanger 100 for greater cooling efficiency. A coolant distribution unit (“CDU”) 116 supplies the rear-door heat exchanger 100 with a chilled liquid coolant, such as water. The CDU 116 pumps the chilled coolant through a supply hose 102 to an inlet manifold 122, which distributes the chilled coolant throughout numerous serpentine tubes in the rear-door heat exchanger 100 to the outlet plenum 124. The numerous serpentine tubes are in direct thermal contact with internal heat exchange fins (further discussed in relation to FIG. 5). As heated air exiting the double-wide rack 10 passes through the rear-door heat exchanger 100, the heated air passes over the heat exchange fins, so that heat from the air is transferred to the coolant, thereby cooling the air exiting the rear-door heat exchanger 100. An outlet manifold 124 receives the heated coolant from the tubes and passes the heated coolant to the CDU 116 through a return hose 104. The CDU 116 chills the received coolant before returning the coolant to the rear-door heat exchanger 100 through the supply hose 102. The various aspects of the rear-door heat exchanger 100 discussed herein allow the rear-door heat exchanger 100 to remove substantially 100% of the heat that was previously added to the air by the various rack-mounted modules.

FIG. 2 is a schematic partial assembly view of the double-wide rack 10 illustrating how various modules may be supported on the double-wide rack 10. Multiple chassis such as exemplary chassis 32, 34 are supportable on longitudinal rails 40 at different vertical locations to establish numerous “chassis bays.” The rails 40 are spaced apart at different vertical positions to accommodate a number of chassis of the same or different sizes, and the vertical positions of the rails 40 may be individually adjustable. The first exemplary 2 U (two unit) chassis 32 is shown being inserted into a chassis bay 36 of the high-power zone 16 on the left. A second, 3 U (three unit) chassis 34 is shown being inserted into a chassis bay 38 in the high-power zone 18 on the right. The rails 40 are spaced at a first vertical distance to accommodate the 2 U chassis 32 and a second, larger vertical distance to accommodate the 3 U chassis 34. The lower left side rail 40 of each chassis bay secures an AC electrical connector 48 arranged to blind dock with a chassis power supply. The electrical connector 48 is aligned with a mating connector on the power supply so that complete insertion of the chassis into the respective chassis bay completes the connection and supplies electrical power to the respective chassis power supply. No access from the rear 14 of the double-wide rack 10 is necessary to complete this connection.

Each chassis has a number of openings (typically more than one) referred to as “module bays” for receiving a corresponding number of modules. The 2 U chassis 32 is shown independently receiving two 1 U compute modules 46. The 3 U chassis 34 is shown receiving a compute module 46 in a lower 1 U module bay and has already received twelve 3.5 inch disk drives 44 that are installed into drive bays that occupy the equivalent of 2 U space and which are a permanent part of the chassis 34. Additional chassis having additional modules bays may be mounted on the double-wide rack 10, such that the double-wide rack 10 supports numerous modules. The modules may be selectively interconnected with cable connections from the front 12 of the double-wide rack 10. Some of the modules, such as compute modules and hard drive modules disposed in the high-power bays 16, 18, may be interconnected within their common chassis.

A plurality of low-power module bays 50 are provided in the low-power zone 20 immediately adjacent to the high-power zone 16, and in the low-power zone 22 immediately adjacent to the high-power zone 18. The low-power module bays 50 suitably receive various low-power modules, such as network switches and PDUs. The close positioning of the low-power zones 20, 22 to the respective high-power zones 16, 18 facilitates cable connections between the low-power modules and the high-power modules they support. For example, a network switch may be positioned in one of the low-power module bays 50 in the low-power zone 20 and connected to a compute module 46 at the same vertical position in the high-power zone 16 to connect that compute module 46 to the network switch. The close positioning of the network switch to the compute module 46 minimizes the physical length of network connections made between the network switch and the compute module 46, and avoids interfering with other modules located elsewhere in the double-wide rack 10. Having these cables and connections in the front of the rack makes configuration easier and does not require access to the back of the rack. Still further, positioning the lower power module bays 20, 22 consistently to one side of the respective high-power zones 16, 18 make cabling even more convenient and manageable. Each chassis in the double-wide rack 10 may include a fan assembly for generating airflow through that chassis.

The top of the 2 U chassis 32 is partially cut-away to reveal an on-board fan assembly 60 having a set of four fans 58. The fan assembly 60 may be directly powered and controlled by the power supply for the chassis 32 according to thermal sensor data passed to it from the compute module 46. Although the number of fans may vary, the 2 U chassis 32 can accommodate larger diameter fans than a 1 U chassis due to the 2 U height. An even larger chassis can support even larger fans. For example, the 3 U chassis 34 may include four fans (not shown) that may each be larger than the fans 58 in the 2 U chassis of FIG. 3. Even larger diameter fans could be used in larger chassis (e.g. a 4 U chassis and larger), within the limits of the chassis physical dimensions. The use of larger fans may provide more efficient air flow, even in cases wherein the larger fan diameter dictates the use of fewer fans. The fan assembly in each chassis generates airflow for cooling that chassis. Collectively, the numerous fans included among the plurality of chassis in the double-wide rack 10 provide sufficient airflow to cool the individual chassis and also to move the air through the rear-door heat exchanger 100 to cool the airflow exiting the double-wide rack 10 without the use of any additional, outboard fans.

FIG. 3 is a rear elevation view of the double-wide rack 10 with a partially cut-away view of the rear-door heat exchanger 100 revealing the double-wide arrangement of the modules, with the two vertically-oriented high-power zones 16, 18 and the two adjacent, vertically-oriented low-power zones 20, 22. A hinged edge 110 of the rear-door heat exchanger 100 is pivotably connected to the double-wide rack 10 by a hinge 107. The rear-door heat exchanger 100 can be opened from a free end 112 to provide access to some of the equipment in the double-wide rack 10 from the rear 14 of the double-wide rack 10. However, the rear-door heat exchanger 100 typically remains closed in a cooling mode so that substantially all of the airflow driven through the double-wide rack 10 from the front 12 to the rear 14 exits through the rear-door heat exchanger 100. It should be recognized that the rear door heat exchanger can be hinged on either side of the rack, including the left side (not shown), but the arrangement of the rack zones would preferably also be reversed to maintain the beneficial access and cooling relationships described above.

The rear-door heat exchanger 100 spans the entire width W and height H of the double-wide rack, to maximize the cross-sectional heat-exchange surface area (i.e., H×W) through which airflow exits the rack 10. The cross-sectional heat-exchange surface area is equal to or greater than the combined cross-sectional area of the high-power zones 16, 18. For example, the high power zones 16, 18 may each have a width of about 451.5 mm (a combined width of about 903 mm), while the width W of the rear-door heat exchanger 100 may be about 999 mm. Also, the width W may be more than twice the effective width of a heat exchanger on a “single wide” rack having a single vertical column of high-power modules. The double-wide rear-door heat exchanger 100 provides more than twice the width of a single-wide door, because the effective cooling area of the heat exchanger does not include the door frame, and the door frame occupies a comparatively smaller proportion of the door. This increased area contributes to improved cooling capacity as compared with conventional means of cooling racks. The coolant flow rate through the rear-door heat exchanger 100 may be increased accordingly (e.g. doubled) to account for the increased width of the rear-door heat exchanger 100. The rear-door heat exchanger 100 may remove up to 100% or more of the quantity of heat that was added to the airflow by the modules in the double-wide rack 10.

A coolant circulation system provides a continuous supply of chilled coolant from the CDU 116 to the rear-door heat exchanger 100 without interfering with airflow through the large heat exchange surface area provided by the rear-door heat exchanger 100. In particular, the hoses 102, 104 are routed horizontally under a lower edge of the rear-door heat exchanger 100. The horizontally-routed portion of the hose 102 is coupled to an inlet manifold 122 with a hose coupler 103 and the horizontally-routed portion of the hose 104 is coupled to an outlet manifold 124 with a hose coupler 104, such that a flow axis of each of the hose couplers 103, 105 is substantially horizontal and substantially perpendicular to the axis of the hinge 107. As a result, the hoses 102, 104 avoid interference with airflow through the rear-door heat exchanger 100, to prevent any airflow losses (e.g. airflow impedance or airflow leakage) that may otherwise result if the hoses 102, 104 were routed in front of some of the modules. Routing the hoses 102, 104 along the lower edge of the rear-door heat exchanger 100 also allows heat exchange fins to be positioned along the entire height of the rear-door heat exchanger 100 (from top to bottom) such that 100% of the airflow through the rack 10 passes over the heat exchange fins. By contrast, a conventional rack-mounted heat exchanger, having heat exchange fins that do not extend the entire height of the rack, would otherwise require a “bypass zone” for exhausting air from some of the modules so that airflow impedance is not too high.

The relatively narrow diameter of the hoses 102, 104 as compared with their length allows the hoses 102, 104 to be positioned out of the way of the airflow, between the lower edge of the rear-door heat exchanger 100 and the floor 125, with ample clearance under the rack 10 for routing. The hose couplers 103, 105 are optimally positioned in the “shadow” of the base pan of the rack 10 so they do not interfere with forktruck access below, or interfere with air flow above in the rack 10. Also, positioning hoses 102, 104 low is desirable in the contingency that a leak occurs during use or if coolant spills while connecting or disconnecting the hoses 102, 104. By contrast, if the quick-connect couplers 103, 105 were vertically oriented, the quick-connect couplers 103, 105 would have to be positioned significantly higher, such as eight inches or more above the floor 125, to provide access for personnel to connect the hoses 102, 104. Such an elevated positioning of the quick-connect couplers 103, 105 would interfere with airflow through the rear-door heat exchanger 100 by shortening the effective height of the heat-exchange fins, by occupying horizontal space that may otherwise have been provided for the heat exchange fins, or by increasing the required depth of the rear-door heat exchanger 100. Thus, the horizontal routing of the hoses 102, 104 to avoid airflow losses contributes to maximizing the cooling efficiency of the rear-door heat exchanger 100.

The inlet manifold 122 and outlet manifold 124 are positioned at the free end 112 of the rear-door heat exchanger 100, opposite the hinged end 110. This location of the inlet manifold 122 and outlet manifold 124 places the manifolds 122, 124 next to the low-power zone 22, and spaced from any of the high-power zones 16, 18. Less heat is generated adjacent to the low-power zone 22 than at the high-power zones 18, 20. Thus, the inlet manifold 122 and outlet manifold 124 or additional structure (e.g. mounting brackets) used to support the inlet manifold 122 and outlet manifold 124 on the rear-door heat exchanger 100 could protrude slightly into the airflow through the rear-door heat exchanger 100 without interfering with the airflow directly in front of either of the two high-power zones 18, 20. By avoiding interference with the airflow in front of the high-power zones 18, 20, any incidental protrusion of the inlet manifold 122, outlet manifold 124, or supporting structure thereof into the airflow exiting the rear-door heat exchanger 100 is unlikely to significantly affect the cooling performance of the rear-door heat exchanger 100, or to substantially increase airflow impedance to the high power zones 16, 18.

FIG. 4 is a perspective view of the rear-door heat exchanger 100 from below, further illustrating the interconnection of the hoses 102, 104 with the rear-door heat exchanger 100. The supply hose 102 and return hose 104 are coupled to the inlet manifold 122 and the outlet manifold 124 along the bottom edge of the rear-door heat exchanger 100 via a corresponding pair of “quick-connect” couplers 103, 105. The quick connect coupler 103 places the supply hose 102 in fluid communication with the inlet manifold 122, and the quick-connect coupler 105 places the return hose 104 is in fluid communication with the outlet manifold 124. The quick-connect couplers 103, 105 are positioned with their flow axis generally oriented horizontally to accommodate the horizontal routing of the hoses 102, 104 without any sharp elbows or sharp bends at the hoses 102, 104.

FIG. 5 is a sectioned view of a portion of the fin tube assembly 114. The fin tube assembly 114 includes a plurality of tubes 126 (only one complete tube 126 is shown) that pass through the rear-door heat exchanger 100 in a serpentine fashion, circulating chilled coolant from the inlet manifold 122 to the outlet manifold 124 under the force of an external pump (not shown) within the CDU 116. The inlet 122 and outlet manifolds 124 include multiple parallel tube branches having their own inlets and outlets. The tubes are in direct thermal communication and contact with a plurality of heat exchange fins 128 that collectively provide a large surface area in contact with the airflow that passes through the rear-door heat exchanger 100.

FIG. 6 is a plan view of the double-wide rack 10 with the rear-door heat exchanger 100 in a closed position, further illustrating the routing of the hoses 102, 104. The rear-door heat exchanger 100 is flush with the rear 14 of the double-wide rack 10, and the direction of the airflow through the fin tube assembly 114 is generally perpendicular to the plane of the rear-door heat exchanger 100, as generally indicated by the arrows 130. The floor in this embodiment is a raised floor 127 of a datacenter, and a cable access opening 140 may be provided in the raised-floor 127. Various power and signal cables may be routed underneath the raised floor 127 and up through the cable access opening 140 to the rack. The hoses 102, 104 may also be routed through the cable access panel 140, without creating separate openings for the hoses 102, 104 in the raised floor 127 that might interfere with casters on the rack 10. The double-wide rack 10 provides plenty of room under the generally rectangular base pan 149 of the rack 10 for the hoses 102, 104 to be routed underneath the rack 10. The hoses 102, 104 are routed from the CDU 116 to the quick-connect couplers 103, 105 along a portion of the perimeter of the rack 10, without extending appreciably outside the “shadow” of the base pan 149 without any sharp bends that might otherwise reduce the flow of coolant through the hoses 102, 104.

FIG. 7 is a plan view of the double-wide rack 10 with the rear-door heat exchanger 110 pivoted to an open position. The rear-door heat exchanger 100 may be opened like a door from the free edge 112 opposite the hinged edge 110, by pivoting the rear-door heat exchanger 100 about the axis of the hinge 107. The rear-door heat exchanger 100 may be opened, for example, to access some components of the double-wide rack 10 from the rear 14 of the double-wide rack 10. The positioning of the inlet and outlet manifolds 122, 124 at the free end 112 opposite the hinged end 110 minimizes bending and flexure of the hoses 102, 104 near the hinged end 110 as the rear-door heat exchanger 100 is opened. Because the hoses 102, 104 pass near and perpendicular to the axis of the hinge 107, the hoses 102, 104 may flex gently and easily as the rear-door heat exchanger 100 is pivoted open and closed about the axis of the hinge 107. This placement of the inlet and outlet manifolds 122, 124 thereby promotes a long service life for the hoses 102, 104 and a steady flow of coolant through the hoses 102, 104.

FIG. 8 is a rear elevation view of the rack 10 illustrating an optional routing of the hoses 102, 104 from the ceiling 131 (instead of from a raised floor) to the rack 10. The hoses 102, 104 are routed along the hinged edge 110 of the rear-door heat exchanger 100, near the axis of the hinge 107, and make a gentle bend 152 so that the hoses can extend underneath the rear-door heat exchanger 100 to be routed perpendicular to the axis of the hinge 107 to the quick-connect couplers 103, 105. Routing the hoses 102, 104 from the ceiling 131 allows the rack 10 to be used on a less expensive slab floor 129 rather than on the generally more expensive and complicated raised-floor of FIGS. 6 and 7.

FIG. 9 is a rear-elevation view of the rack 10 illustrating an alternative routing of the hoses 102, 104 from the ceiling 131. The quick-connect couplers 103, 105 are provided at the top of the rack 10, for coupling the supply hose 102 to the inlet manifold 122 and coupling the return hose 104 to the return manifold 124. This may significantly shorten the length of the hoses 102, 104 as compared with the embodiment of FIG. 8. The hoses 102, 104 are routed along an upper edge of the rack 10 to approximately the hinge 107, where the hoses 102, 104 make a gentle bend and are routed upwardly to the ceiling 131. The portion of the hoses 102, 104 running upwardly to the ceiling 131 is in general alignment with the axis of the hinge 107, so that the rear-door heat exchanger 100 may be opened at the hinge 107 with minimal disturbance to the hoses 102, 104.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, components and/or groups, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The terms “preferably,” “preferred,” “prefer,” “optionally,” “may,” and similar terms are used to indicate that an item, condition or step being referred to is an optional (not required) feature of the invention.

The corresponding structures, materials, acts, and equivalents of all means or steps plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but it not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. 

1. A rack assembly for cooling a computer system, comprising: a rack providing support for one or more columns of heat-generating electronic devices and device fans for moving air from an air inlet side of the rack through the devices and through an air outlet side of the rack; a unitary door having a support frame spanning the air outlet side of the rack and a hinge pivotally coupling the door to a rear vertical edge of the rack, wherein the unitary door includes an air-to-liquid heat exchanger panel spanning an air outlet passage inside the support frame so that substantially all of the air passing through the air outlet side of the rack must pass through the heat exchanger panel; and a coolant circulation system including a coolant distribution unit for chilling liquid coolant, a supply hose placing the coolant distribution unit in fluid communication with an inlet manifold of the air-to-liquid heat exchanger, and a return hose placing an outlet manifold of the air-to-liquid heat exchanger in fluid communication with the coolant distribution unit, wherein the supply hose and return hose are routed outside the cross-sectional area of the air outlet passage.
 2. The rack assembly of claim 1, wherein the one or more columns of heat-generating electronic devices comprise a first set of one or more columns of modular heat-generating electronic devices that each have a minimum power consumption greater than a selected power threshold and a second set of one or more columns of modular heat-generating electronic devices that each have a maximum power consumption less than the selected power threshold.
 3. The rack assembly of claim 2, wherein the air outlet passage has a cross-sectional area that is substantially equal to or greater than the combined cross-sectional area of the first set of one or more columns of modular heat-generating electronic devices.
 4. The rack assembly of claim 1, further comprising: a first hose coupler configured for removably coupling the supply hose to the supply manifold and a second hose coupler configured for removably coupling the return hose to the return manifold, wherein the first and second hose couplers are secured near a lower edge of the rack or the door with a flow axis of each of the first and second hose couplers positioned substantially horizontally and substantially perpendicular to the axis of the hinge.
 5. The rack assembly of claim 1, wherein the first and second hose couplers comprise quick-connect type hose couplers.
 6. The rack assembly of claim 1, wherein the inlet and outlet manifolds extend along a vertical side of the support frame opposite the hinged coupling.
 7. The rack assembly of claim 1, wherein the supply hose is routed from the ceiling to the inlet manifold and the return hose is routed from the outlet manifold to the ceiling.
 8. The rack assembly of claim 7, wherein the rack is positioned on a slab floor.
 9. The rack assembly of claim 7, wherein the supply hose is coupled to the inlet manifold at the top of the rack and the return hose is coupled to the outlet manifold at the top of the rack.
 10. The rack assembly of claim 1, wherein the supply hose is routed through a raised floor to the inlet manifold and the return hose is routed from the outlet manifold through the raised floor.
 11. The rack assembly of claim 10, further comprising a cable access opening in the raised floor, wherein the supply hose and return hose are routed from underneath the floor, through the cable access opening, to the air-to-liquid heat exchanger, and a plurality of electronic cables are also routed from underneath the floor, through the cable access opening, to the heat-generating electronic devices supported on the rack.
 12. The rack assembly of claim 1, wherein the heat-generating electronic devices comprise a plurality of compute modules each having a motherboard and one or more processors, and wherein the supply hose and return hose are spaced at least twelve inches from any of the compute modules.
 13. The rack assembly of claim 1, wherein the supply hose and return hose are routed along a perimeter of the rack underneath a base pan of the rack.
 14. The rack assembly of claim 1, wherein the air-to-liquid heat exchanger panel has a capacity that is sufficient to remove all of the heat generated by the electronic devices within the one or more columns of the rack.
 15. The rack assembly of claim 13, wherein the supply hose and the return hose are routed underneath the base pan toward the back of the rack.
 16. The rack assembly of claim 15, wherein the supply hose and the return hose are free to curve uniformly from the front of the rack to the back of the rack.
 17. The rack assembly of claim 1, wherein the one or more columns is at least two columns. 